Piezoelectric resonator devices hermetically seal excitation electrodes of piezoelectric resonator pieces, which are configured to effect piezoelectric resonance. Examples of the piezoelectric resonator devices include crystal oscillators.
Crystal oscillators each include a package defined by a base made of a ceramic material in the form of a box and a single plate lid made of a metallic material. In the internal space of the package, a piezoelectric resonator piece and an IC chip are held by and bonded to the base. The bonding of the base and the lid hermetically seals the piezoelectric resonator piece and the IC chip in the internal space (see, for example, patent document 1).
The crystal oscillator described in patent document 1 uses a base that includes two box bodies of ceramic materials that are layered one on top of one another by integral firing. The piezoelectric resonator piece is mounted on one of the box bodies of the base, while the IC chip is mounted on the other box body. The crystal oscillator includes, on the rear surface (other principal surface) of the base: external terminals to be electrically connected to an external circuit board; and inspection terminals to measure and inspect the properties of the crystal resonator piece. The external terminals and the inspection terminals are formed along the outer periphery of the other principal surface.
Bases made of box bodies of ceramic materials, such as the one shown in patent document 1, are formed by integral firing, and this causes a state in which stress remains in the bases after fired (the stress being referred to as residual stress).
The base with residual stress has a flat surface across a corresponding region corresponding to a wall portion of the base that is in the form of box bodies, whereas other regions have a surface that is convexly or concavely curved due to residual stress (the other regions being, specifically, the regions on the other principal surface corresponding to a cavity on the one principal surface of the base).